Bone void fillers (BVF) are useful as scaffolds for bone healing. A scaffold conducts bone growth over gaps too large for bone to bridge by itself, and can accelerate the rate of bone healing in voids of all sizes. Calcium salts are generally recognized as a class of BVF materials. For instance, calcium phosphate salts are commonly used as they are related to hydroxyapatite (HA), which is a form of calcium phosphate in bone material. Calcium sulfate and calcium carbonate, while not containing phosphate, are also useful as BVF materials.
The bone healing process consists of three major stages: removal of damaged bone debris, growth of new bone, and final bone remodeling. Specialized bone cells called osteoclasts participate in the removal of bone debris and in resorbing mature bone to aid in remodeling. Other cells called osteoblasts grow new bone onto a collagen structure formed by fibroblasts or onto already existing mineralized surfaces. The sequence of bone growth and remodeling is controlled by specific proteins of the general class of growth factors.
Successful scaffold materials generally allow direct attachment of osteoblasts to their surfaces, and the scaffolds are then eventually remodeled and finally replaced by bone. However, the remodeling rate must not be faster than the rate of bone growth or the bone may not heal. Conventional scaffold materials are especially suited to supporting osteoblast activity and are intended to last until bone has fully bridged any gaps.
Non-cemented implants intended for use in bone can also benefit from an osteoconductive surface. Such implants include joint prostheses, screws, plates, spinal stabilizing devices, etc. Such metal or polymeric implants (in particular, prostheses) are often coated with HA to provide a more bone compatible surface. (Even the metal of such an implant can be specified to be more bone compatible by, for example, using a titanium alloy instead of a cobalt chrome alloy.) An HA plasma sprayed coating can facilitate bone on-growth onto the implant with long-term stability occurring as the coating remodels until the bone is attached directly to the implant surface.
Calcium carbonate applied to an HA coating can speed up the natural remodeling process and achieve final stability sooner than otherwise possible. This can be an advantage especially in dental applications where a slightly porous HA coating can more easily become infected, as compared to a coating in a deep internal location such as a hip implant. As a result, rapid remodeling of the HA coating lessens the likelihood of failure due to infection.
A perceived drawback to use of calcium carbonate is the tendency to stimulate osteoclast activity. This stimulation is probably due to the relatively rapid dissolution of calcium carbonate compared to calcium phosphates and the accompanying release of calcium ions. However, as the active osteoclasts break down bone they liberate osteoblast stimulating growth factors from the bone and, in doing so, can actually bring about more rapid bone healing. But calcium carbonate is not as effective in large voids because it is too soluble. The solubility means calcium carbonate may not be stable enough to function as a scaffold long enough for the bone gap to be bridged. As an alternate approach, calcium carbonate granules or powder can be mixed with a more stable, conventional BVF material, but the calcium carbonate component of such mixtures tends to separate and segregate, thereby inducing concerns of the sort outlined above.